The present invention generally relates to generating test signals for verifying communications devices, and more particularly to creating multi-tone test signals for missing tone tests of different types of Digital Subscriber Line (xDSL) devices to allow easy and flexible testing of such devices.
Generally, xDSL devices, and in particular ADSL devices, convey signals over a bandwidth that ranges from 25 kHz to 1.2 MHz. This bandwidth is divided into different bands or channels, each of which is typically 4.3125 kHz wide. For example, an ADSL signal includes approximately 250 different bands, each 4.3125 kHz wide. To convey data, an xDSL modem provides a different carrier frequency for each of its bands, and provides modulated data around each of the carriers.
One test for determining proper operation of an xDSL device, and in particular an ADSL device, is called a xe2x80x9cmissing tonexe2x80x9d test. The missing tone test determines whether a component (e.g., a circuit such as an application-specific integrated circuit or ASIC) within the xDSL device (e.g., an ADSL modem) introduces distortion into the output of one or more of the channels or frequency bands processed by the device during device operation. To perform a missing tone test, an ATE (automatic test equipment) system generates a broad band test signal and applies it to the input of an xDSL device. In response, the device generates an output. The ATE system samples the output and performs a Fast Fourier Transform or FFT of the output signal. During the test, the ATE system systematically removes individual tones (i.e., bands or channels) from the input waveform (i.e., the test signal), and evaluates the resulting FFTs after an xDSL device processes such a test signal. If the removed tones do not appear in the FFTs (i.e., the FFTs have only low level power components at the missing tone locations) then the test passes. In other words, if the broad band test signal that includes one or more missing bands is applied to the xDSL device and the device produces an output signal that contains little or no output at the location of the missing bands, then the test operator may consider the xDSL device as having passed the test (i.e., is functioning properly) since the device introduced little or no distortion into the original broadband test signal. However, if large components or distortion appear at the missing tone locations (e.g., removed bands or channels), the test operator can consider the xDSL device as having failed the missing tone test since a significant component at a missing tone location indicates that circuitry within the xDSL device has introduced distortion into its output at the missing tone locations.
xe2x80x9cCrest Factorxe2x80x9d (also known as xe2x80x9cPARxe2x80x9d or Peak-to-Average Ratio) is a particularly important characteristic of xDSL signals. Crest factor is defined as the maximum amplitude of a test signal, divided by the average (i.e., Route Mean Squared value) of the signal, or:
Crest Factor=Peak Signal/Average Signal.
For performing missing tone tests, manufacturers desire to specify a particular crest factor of the test signal. Then, they are able to claim that their part has a particular distortion or lack thereof (measured using missing tone tests) at a particular crest factor.
It is often exceedingly difficult, using conventional techniques, to maintain a desired crest factor of a test signal as tones (i.e., carrier signal frequencies) are systematically removed for the purpose of performing missing tone tests.
Frequently, in conventional testing techniques, the removal of a tone signal from a test signal significantly changes the crest factor of the resulting test signal. This is because the test signal is typically a composite or summation of sine or cosine waves with coinciding peaks that form a peak value in the test signal over a short duration of the overall period of the test signal. Accordingly, removal of one tone signal can significantly change the peak value and the average value of the resulting composite test signal. For example, removal of a tone signal (e.g., a carrier frequency) from a composite signal formed by ten tone signals (therefore leaving only nine remaining tone signals) may reduce the peak value by approximately ten percent and may also therefore affect the average value of the test signal.
To maintain a constant crest factor of a test signal, test developers using conventional techniques customarily perform the trial and error task of manually and repetitively adjusting the phases of the remaining tones in the test signal, and re-measuring the resulting crest factor of that test signal. This manual and repetitive process can consume significant time. As a test developer removes each tone from the broad band test signal for an xDSL device, in conventional systems, the developer must xe2x80x9cre-balancexe2x80x9d the phases of remaining tones to re-achieve the desired crest factor of the test signal.
The present invention significantly overcomes these and other problems associated with conventional signal generation and device testing techniques used for testing xDSL or other types of data communication devices.
More specifically, according to embodiments of the invention, a method is provided for generating a test signal. Preferably, the test signal is used as a test signal for xDSL devices under test. The method comprises the steps of selecting a set of frequencies (e.g., a range or ranges of tones between a start and stop frequency) for the test signal and selecting frequency sub-groups from the set of frequencies. Next, the method generates a respective sub-group composite signal for each frequency sub-group selected from the set of frequencies and then time shifts each respective sub-group composite signal in relation to other sub-group composite signals. Finally, the method generates the test signal by summing each respective time shifted sub-group composite signal to produce the test signal.
In this manner, sub-group composite signals each include a respective peak which when combined with other sub-group composite signals are spread out (e.g., time-shifted or delayed) across the test signal. During missing tone tests, if a tone frequency is removed from a sub-group of frequencies, the sub-group composite signal corresponding to that frequency may be affected, but the other sub-group composite signals will remain largely unaffected by the missing tone frequency. Since the other sub-group composite signals contain peaks at the desired value (as explained below), the test signal as a whole is less affected by the missing tone. Accordingly, removal of a signal from one of the sub-groups has a small effect on the average value and generally little or no effect on the peak value of the resulting test signal.
In another embodiment of the invention, the step of selecting a set of frequencies for the test signal includes the steps of determining at least one start frequency and at least one stop frequency defining-the set of N frequencies to be included in the test signal. The method embodiments can determine start and stop frequencies, for example, by obtaining such frequencies from a test developer (e.g., a person) that controls the signal generator configured to carry out the method embodiments of the invention. The method also includes the step of determining any intermediate frequencies to be included in the test signal between the at least one start frequency and the at least one stop frequency that occur at frequency intervals equal to a desired tone spacing of frequencies for the test signal. Such intermediate frequencies include all frequencies between the start and stop frequency, or, may include a discontinuous range or frequencies. The range may be discontinuous due to a test developer selecting missing tone frequencies to be omitted from the set of frequencies used for the test signal. In this manner, a selection of missing tone frequencies that the test developer provides between the start and stop frequencies defines the remaining set of frequencies to be used in the test signal.
In yet another embodiment of the invention, the step of determining at least one start frequency and at least one stop frequency defining the set of N frequencies to be included in the test signal includes the steps of defining at least one missing tone frequency. As noted above, a test developer may specify any missing tone frequencies to be omitted between the start and stop frequencies in order to define the set of frequencies to be included in the test signal, or, a frequency selector (to be explained later) may be preprogrammed with a predetermined set of missing tone frequencies which are to be omitted or removed from the set of frequencies ranging from the start to the stop frequency. In this embodiment then, the missing tone frequency defines a frequency to be omitted from the set of N frequencies in order to produce a test signal to be used for missing tone testing of a data communications device, such that the set of N frequencies for the test signal includes a plurality of ranges of frequencies each beginning with a start frequency and ending with a stop frequency. Collectively, the plurality of ranges of frequencies defined in this manner define the set of frequencies to be used in the test signal.
In another embodiment, the step of determining any intermediate frequencies includes the step of selecting frequencies equal to carrier signals that match harmonics of a tone spacing frequency between the at least one start frequency and the at least one stop frequency. In cases in which the test signal to be used to test digital subscriber line devices, the tone spacing frequency is the frequency spacing of tones, bands or channels in the range of frequencies that a digital subscriber line device commonly uses in operation.
In still a further embodiment of the invention, the step of selecting frequency sub-groups from the set of frequencies includes the steps of determining a crest factor for the test signal and determining a desired peak value K for the test signal based on the crest factor for the test signal. The system of the invention may determine the crest factor, for example, by prompting the test developer for a desired crest factor or the system may be preprogrammed with the desired crest factor. This method embodiment also includes the step of dividing the set of frequencies for the test signal into frequency sub-groups, wherein at least one frequency sub-group contains K frequencies selected from the set of frequencies. In this manner, the set of frequencies to be used to the test signal is divided into frequency subgroups each containing substantially K (or fewer) frequencies.
In another embodiment, the step of dividing includes the steps of dividing a number N of frequencies, representing all frequencies in the set of frequencies for the test signal, by the desired peak value K to determine a number of frequency sub-groups and selecting individual frequencies from the set of frequencies to be included in each frequency sub-group. In this embodiment, the frequencies selected for each frequency sub-group are substantially selected evenly from across the N frequencies in the set of frequencies for the test signal, such that each frequency sub-group has substantially K frequencies evenly distributed across the N frequencies in the set of frequencies for the test signal. In other words, the system of the invention determines a number of frequency sub-groups by dividing the total number of frequencies by the desired peak value K, and then assigns K frequencies to each frequency sub-group. The frequencies in each subgroups are preferably distributed substantially evenly across the entire range of frequencies in the set of frequencies for the test signal.
According to another embodiment of the invention, the step of determining a desired peak value K for the test signal includes the step of calculating the desired peak value K by multiplying the crest factor for the test signal by the average root mean square of a number N of frequencies, representing all frequencies in the set of frequencies for the test signal.
In another embodiment, the set of frequencies for the test signal is a discontinuous set of frequencies within a frequency range used for testing digital subscriber line devices and frequencies not included in the discontinuous set of frequencies are test tone frequencies that are intentionally omitted in order to perform missing tone tests on digital subscriber line devices. As noted above, the test signal to be used for missing tone testing digital subscriber line devices by selecting a set of frequencies as explained above and then establishing a set of missing tone frequencies which are removed from the set of frequencies. Alternatively, the missing tone frequencies may first be specified (e.g., by a test developer) after which the start and stop frequency spanning the range of missing tone frequencies may be specified us defining the set of frequencies which excludes the missing tone frequencies. The point is that the set of frequencies used for the test signal of the invention when performing missing tone testing includes a range of frequencies suitable for use in testing digital subscriber line devices absent any specified missing tone frequencies.
In another embodiment of the invention, the step of generating a respective sub-group composite signal for each frequency sub-group includes the step of, for each respective frequency sub-group, summing together all frequencies within that frequency sub-group to produce a respective sub-group composite signal for that frequency sub-group. In this manner, once the frequency sub-groups are established, the sub-group composite signal for each frequency subgroups is generated according to the techniques explained herein. There is thus one sub-group composite signal for each frequency sub-group.
In another embodiment, the step of time shifting each respective sub-group composite signal imposes a predetermined delay on each respective sub-group composite signal. In one embodiment, the predetermined delay is calculated by dividing a period of the test signal by a number of frequency sub-groups squared to obtain a delay increment D. The delay increment essentially represents an amount of time in which to delay (or space) each sub-group composite signal so as to evenly distribute peak portions of each sub-group composite signal evenly across an entire period of the resulting test signal. In this embodiment, for frequency sub-groups numbered 0 through M, the predetermined delay imposed on each sub-group is equal to the delay increment D times the number 0 through M of that sub-group, such that each respective sub-group composite signal is distributed evenly within the test signal upon performing the step of generating the test signal. Thus if there are four frequency sub-groups, the first is numbered zero and the last is numbered three. The first sub-group is delayed by a delay increment have a value of zero (zero times the delay increment), the second is delayed by the delay increment exactly (one times the delay increment), the third is delayed by two times the delay increment, and the fourth is delayed by three times the delay increment.
In still another embodiment, each respective sub-group composite signal includes a peak region which occurs over a peak interval of a period of that sub-group composite signal the step of time shifting each respective sub-group composite signal offsets the peak region of each sub-group composite signal from other peak regions of other sub-group composite signals, such that the step of generating the test signal generates a test signal having multiple peak regions, each corresponding to a peak region of a sub-group composite signal which are offset from one another. Accordingly, since the frequency sub-groups are initially in-phase with each other and each has a respective peak value, the step of time-shifting the frequency sub-groups (e.g., by calculating a delay increment as in the aforementioned embodiment) causes the peak values of each frequency sub-group to be shifted out of phase with each other by an even amount of spacing across the period of these frequency sub-groups.
Accordingly, the resulting test signal having multiple peak regions evenly distributed therein can be used for missing tone testing and when additional or different missing tones are selected each time the test signal is generated in this manner, only those peak regions in the test signal corresponding to sub-group composite signals from which the missing tones are omitted have their peaks effected (e.g., lowered slightly). In other words, if a test signal is generated with a first set of missing tones from one particular frequency sub-group, the peak value for that particular frequency sub-group might be slightly effected (i.e., lowered) but the other peak signals for other frequency sub-groups would remain largely unaffected when all of the delayed (i.e., time-shifted) sub-group composite signals are collectively summed together to generate the test signal having multiple peaks. Subsequently, when a second missing tone test is to be performed using a second set of missing tone frequencies, the second set of missing tone frequencies might, perhaps, only effect the peak value of a different frequency sub-group. Accordingly, the other peak values corresponding to frequency sub-groups from which no missing tones are omitted are largely unaffected and thus the resulting test signal includes many peaks that attain and remain at a maximum value.
The unique test signal generation techniques summarized above produce test signals that have an increased chance of being sampled by a device under test at a maximum peak value (due to the multiple peak values) during missing tone testing, even though one or more peak values within test signal may correspond to a sub-group of frequencies from which missing tones are omitted.
Other embodiments of the invention include methods for testing a device. In one such embodiment, a method comprises the steps of grouping a plurality of carrier signals into sub-groups of carrier signals and generating a test signal including multiple peaks. Each peak corresponds to a respective sub-group of carrier signals. The method provides the test signal to a device under test. In this manner, the unique technique of generating the test signal is used to test a device, such as a digital subscriber line device.
Another embodiment, the method includes the step of selecting a carrier signal to omit from a sub-group of carrier signals, such that a peak in the test signal corresponding to the sub-group of carrier signals from which the carrier signal is omitted reflects the omission of the carrier signal, while other peaks corresponding to other sub-groups of carrier signals are not significantly affected by the omission of the carrier signal. As summarized above, the signal generation techniques of the invention allow a test developer to develop the test signal having such multiple peak signals without having to adjust various signal generation parameters for multiple iterations, for example, of missing tone tests in which different sets of tones are omitted during the generation of each test signal.
In yet another embodiment, the method includes the steps of receiving an output signal from the device under test. The output signal is based on the test signal. The method also performs an analysis of the output signal to determine an effect that the device under test has on the test signal in relation to the carrier signal selected for omission from the sub-group of carrier signals.
Other embodiments of the invention relate to signal generation mechanisms. One such embodiment provides a signal generation apparatus for generating a test signal. The signal generation apparatus comprises a frequency selector selecting a set of frequencies for the test signal and selecting frequency sub-groups from the set of frequencies. Generally, the frequency selector performs the method embodiments above to allow, for example, the test developer to interact with a signal generator configured according to the embodiment to specify the frequencies to be used within the test signal into further specify, if performing missing tone testing, any missing tones to be omitted from the range of frequencies used in the set of frequencies in the test signal.
This embodiment also includes at least one sub-group summer circuit coupled to the signal generation apparatus to receive the frequency sub-groups. The sub-group summer circuit(s) generate respective sub-group composite signals for the frequency sub-groups which it receives. The sub-group summer circuits therefore produce sub-group composite signals for each frequency sub-group defined by the frequency selector. A sub-group summer circuit is generally a signal summation circuit which is capable of summing multiple frequencies together to provide a single output signal. There may be a single sub-group summer circuit to which each frequency sub-group is provided in order for that single sub-group summer circuit to generate a corresponding sub-group composite signal or, alternatively, the signal generation apparatus may include multiple sub-group summer circuit""s each responsible for summing the various frequencies within one or more frequency sub-groups into respective sub-group composite signals.
The signal generation apparatus further includes at least one delay shifter circuit coupled to the signal generation apparatus to receive at least one sub-group composite signal generated by the sub-group summer circuit(s). The delay shifter circuit time shifts or delays at least one sub-group composite signal in relation to other sub-group composite signals such that the sub-group composite signals for the frequency sub-groups become delayed (i.e., time-shifted) sub-group composite signals. Delay shifter circuits can delay sub-group composite signals by a delay that can range from zero to a maximum predetermined time period. Also included is a composite signal summer coupled the signal generation apparatus to receive and sum each delayed sub-group composite signal to produce the test signal.
In another embodiment, the frequency selector includes a means for determining at least one start frequency and at least one stop frequency defining the set of N frequencies to be included in the test signal. Such means can include, for example, an interface to allow a test developer to input (e.g., via a keyboard or other input mechanism) the starting stop frequencies. The frequency selector also includes a means for determining any intermediate frequencies to be included in the test signal between the at least one start frequency and the at least one stop frequency that occur at frequency intervals equal to a desired tone spacing of frequencies for the test signal. Such means can be, for example, processing circuitry or logic instructions (e.g., software code) that can calculate each required frequency for the test signal between the start and stop frequencies specified by the test developer.
In another embodiment, the means for determining at least one start frequency and at least one stop frequency defining the set of N frequencies to be included in the test signal includes a means for defining at least one missing tone frequency. The missing tone frequency defines a frequency (or multiple frequencies) to be omitted from the set of N frequencies in order to produce a test signal to be used for missing tone testing of a data communications device, such that the set of N frequencies for the test signal includes a plurality of ranges of frequencies each beginning with a start frequency and ending with a stop frequency. The means for defining at least one missing tone frequency may be, for example, the user interface that prompts a test developer to input one or more missing tone frequencies to be omitted from a test signal. Alternatively, such a means to the processing circuitry or logic instructions which predefined which missing tones are to be omitted from a test signal.
In another embodiment, the means for determining any intermediate frequencies includes means for selecting frequencies equal to carrier signals that match harmonics of a tone spacing frequency between the at least one start frequency and the at least one stop frequency. Such a means may be, for example, processing circuitry or logic instructions which perform on a processor within the signal generation apparatus to calculate the intermediate frequencies.
In another embodiment, the frequency selector includes a means for determining a crest factor for the test signal. Such a means may be, for example, an interface which prompts the test developer to enter a desired crest factor for the test signals or may be processing circuitry or logic instructions which perform on a processor within the signal generation apparatus to compute the crest factor based on other information. Likewise, this embodiment also provides a means for determining a desired peak value K for the test signal based on the crest factor for the test signal, such as processing circuitry or logic instructions, and includes a means for dividing the set of frequencies for the test signal into frequency sub-groups, wherein at least one frequency sub-group contains K frequencies selected from the set of frequencies. It is to be understood by those skilled in the art that many different types of circuitry and/or computer systems encoded logic instructions can be used to perform the collection of data and signal processing calculations explained herein.
In another embodiment, the means for dividing includes a means for dividing a number N of frequencies, representing all frequencies in the set of frequencies for the test signal, by the desired peak value K to determine a number of frequency sub-groups and a means for selecting individual frequencies from the set of frequencies to be included in each frequency sub-group. The frequencies selected for each frequency sub-group are substantially selected evenly from across the N frequencies in the set of frequencies for the test signal, such that each frequency sub-group has substantially K frequencies evenly distributed across the N frequencies in the set of frequencies for the test signal. Such means include processing circuitry or logic instructions which perform these operations within the signal generation apparatus configured in accordance with the invention.
In yet another embodiment, the means for determining a desired peak value K for the test signal calculates the desired peak value K by multiplying the crest factor for the test signal by the average root mean square of a number N of frequencies, representing all frequencies in the set of frequencies for the test signal.
In another embodiment, the set of frequencies for the test signal is a discontinuous set of frequencies within a frequency range used for testing digital subscriber line devices and frequencies not included in the discontinuous set of frequencies are test tone frequencies that a test developer intentionally omits in order to perform missing tone tests on digital subscriber line devices.
In another embodiment, the sub-group summer circuit(s) generate a respective sub-group composite signal for each frequency sub-group by summing together, for each respective frequency sub-group, all frequencies within that frequency sub-group to produce a respective sub-group composite signal for that frequency sub-group. The sub-group summer circuits to be any type of signal processing circuitry, software, or combination thereof which is capable of processing signals in this manner.
In still another embodiment, the at least one delay shifter circuit imposes a predetermined delay on each respective sub-group composite signal. As explained above, the predetermined delay offsets the peak intervals or regions within each sub-group composite signal such that when the signals are combined into the final test signal, the test signal includes multiple peak regions.
In another embodiment, the delay shifter circuit(s) calculates the predetermined delay by dividing a period of the test signal by a number of frequency sub-groups squared to obtain a delay increment D. In this case, for frequency sub-groups numbered 0 through M, the predetermined delay imposed by the at least one delay shifter circuit on each sub-group is equal to the delay increment D times the number 0 through M of that sub-group, such that each respective sub-group composite signal is distributed evenly within the test signal upon operating the composite signal summer to generate the test signal.
In another embodiment of the signal generation apparatus, each respective sub-group composite signal generated by the sub-group summer circuit(s) includes a peak region which occurs over are peak interval of a period of that sub-group composite signal and the means for time shifting each respective sub-group composite signal offsets the peak region of each sub-group composite signal from other peak regions of other sub-group composite signals, such that the composite signal summer generates a test signal having multiple peak regions, each corresponding to a peak region of a sub-group composite signal which are offset from one another.
Other embodiments of the invention include mechanisms for testing devices, such as digital subscriber line devices. In one such embodiment, a system is provided for testing a device. The system includes a test signal generator that groups a plurality of carrier signals into sub-groups of carrier signals and generates a test signal including multiple peaks, each peak corresponding to a respective sub-group of carrier signals. The test signal generator includes a test interface providing the test signal to a device under test. As explained above, since the peaks in the test signal correspond to sub-groups of carrier signals, the test signal can have multiple peak regions and is thus more likely to be sampled a peak value by a device under test.
In another embodiment, the system includes a test developer interface allowing a test developer to select a carrier signal(s) to omit from a sub-group(s) of carrier signals, such that a peak in the test signal corresponding to the sub-group of carrier signals from which the carrier signal is omitted reflects the omission of the carrier signal, while other peaks corresponding to other sub-groups of carrier signals are not significantly affected by the omission of the carrier signal. Such an embodiment allows a test developer to select different carrier signal(s) for omission from one or more sub-groups upon successor uses of the system to generate multiple test signals. Each time the test developer selects one or more of the carrier signals for omission, the system of the invention creates a test signal which remains in the desired peak value without requiring the test developer to constantly adjust attributes of carrier signals to bring the test signal up to the desired peak value, as is done in conventional systems.
In another embodiment, the test interface receives an output signal from the device under test and the output signal is based on the test signal. For example, the device under test processes the test signal generated in accordance with the invention to produce the output signal. The device under test may be, for example, the digital subscriber line device and the test signal is the missing tone test signal provided to the device under test to perform missing tone testing on the device under test. A test signal analyzer coupled to receive the output signal performs an analysis of the output signal to determine an effect that the device under test has on the test signal in relation to the carrier signal selected for omission from the sub-group of carrier signals.
Other embodiments of the invention include test signals themselves which are embodied within a carrier medium such as one or more radio frequency waves propagated within carrier mediums that can transport radio frequency waves, or one or more electrical impulses with a conductive medium such as one or more electrical wires or cables, or as one or more optical waves propagated within an optical medium such as a fiber-optic cable or other similar interface.
Specifically, one such embodiment is a propagated test signal for testing a digital subscriber line device. The propagated test signal is transported on a carrier medium such as one of the carrier mediums identified above. The propagated test signal has multiple test signal peak regions. Each test signal peak region corresponds to a peak region of a respective delayed sub-group composite signal. Each respective delayed sub-group composite signal represents a summation of a sub-group of frequencies selected from a set of frequencies selected to be included in the test signal.
In another embodiment, in the aforementioned propagated signal, the set of frequencies selected to be included in the test signal is a distribution of frequencies selected to exclude missing tone frequencies from a range of frequencies used for testing the digital subscriber line device. Such a propagated signal of such an embodiment is a missing tone test signal.
In another embodiment, a propagated test signal is provided for testing a digital subscriber line device. The propagated test signal is transported on a carrier medium. The propagated test signal has multiple test signal peak regions, each test signal peak region corresponding to a peak region of a respective delayed sub-group composite signal, and each respective delayed sub-group composite signal represents a summation of a sub-group of frequencies selected from a set of frequencies selected to be included in the test signal. Also in a propagated test signal of such an embodiment, at least one test signal peak region represents a summation of a missing-tone delayed sub-group of frequencies from which frequencies are omitted for missing tone testing of the digital subscriber line device, while other test signal peak regions represent other delayed sub-group composite signals from which no missing tone frequencies are omitted, such that the other test signal peak regions are substantially unaffected by the omission of frequencies from a missing tone delayed sub-group of frequencies. In this manner, such a signal contains multiple peak regions, some of which (e.g., one or more) may reflect the omission of missing tone frequencies from the test signal, while other peak regions do not substantially reflect the omission of such tones.
The aforementioned embodiments of the invention may be performed, produced or otherwise created or used by mechanisms manufactured and/or sold by Teradyne, Inc. of Boston, Mass.